Gravimetry is a well known method of mapping subsurface geology utilizing potential fields, for resource development purposes. Although gravimetry is well known in the art, its application and use to date have been restricted by cost considerations relating to the cost of determining station elevation. Gravimeter measurements, per se, are largely meaningless unless accompanied by an accurate determination of the relative elevation of each station. To be consistent with the accuracy of modern first order gravimeters such as those manufactured by Scintrex Limited or La Coste, the determination of station elevation traditionally requires the application of optical levelling. Optical levelling is a slow process and is costly for determining the elevation of widely spaced stations (eg. 1 km or more apart), or when applied in rugged or forested areas. By way of contrast, gravimeter readings themselves are fast, typically requiring only 2-3 minutes per station, and they constitute only a minor portion of the total survey cost where optical levelling is employed. Other means of determining station elevation, such as the use of micro-barometric altimeters, result in much lower accuracy (typically .+-.1 m to .+-.3 m in elevation, equivalent to .+-.0.03 mGals to .+-.1 mGals in gravity).
Recent developments in the design of differential GPS (Global Positioning System) receivers has resulted in the ability to obtain high accuracy gravimeter station elevation calculations. For example, Fetherstone and Dentith, "Matters of Gravity: The Search for Gold", GPS World, July 1994, have shown that a standard deviation (SD) of a measurement of 2.2 cm can be realized by such means. In terms of speed and cost, this represents a major advance in the field of gravity surveying where the stations are more than 500 m apart.
The use of helicopters to transport and position gravimeter crews has also been well known in the art for several decades. The efficiency of this practice, however, relies heavily on the availability of suitable landing sites which are conveniently located in proximity to the desired station locations. This, unfortunately, is not always the case, particularly where the stations are located in rugged topography, water covered or in marshland, in closely spaced tree cover, etc. In some areas, there are prohibitions against landing, due to hazards or to protect delicate environmental biosystems, etc.